I would recommend either a second Tri 210L, or a Whappo, Jr. I use the Jr. with the 210L, and it is a great combo. Huge wall of phat, thick, yet articulate sound.

Now, I have also used a Tri 110 with my 210L (both of them stacked on their side), and the sound is quite different. Very punchy, and even growly (I attribute this to the 110 - it's growly in a good way, I think, but it is somewhat unique in this regard for an Accugroove cab). Some others have tried a Tri 112L on top (again, both cabs on their side would probably look the best), and I have no doubt that this would be a great combo.